being defined. Throughout, write to these struct tm members only if
CYGWIN_VERSION_CHECK_FOR_EXTRA_TM_MEMBERS is true.
* libc/strptime.cc: Ditto.
* include/cygwin/version.h (CYGWIN_VERSION_CHECK_FOR_EXTRA_TM_MEMBERS): Define.
(CYGWIN_VERSION_API_MINOR): Bump to 272.